Elucidating the Mechanism of Nucleation near the Gas-Liquid Spinodal

We have constructed, using a variant of umbrella sampling technique, a multidimensional free energy surface of nucleation of the liquid phase from the parent vapor phase near the gas-liquid spinodal as a function of relevant nucleation coordinates. Extensive simulation studies have been performed for the supercooled Lennard-Jones gas and the 3-dimensional Ising model. In particular, we remove the Becker-Doring constraint of having only one growing cluster in the system. Close to the spinodal, the free energy, as a function of the size of the largest cluster, develops surprisingly a minimum at a sub-critical cluster size. It is this minimum at intermediate size that is found to be responsible for the barrier towards further growth of the nucleus at large supersaturation. An alternative free energy pathway involving the participation of many sub- critical clusters is found near the spinodal where the growth of the nucleus is promoted by a coalescence mechanism. Temperature quench trajectory analyses reveal that the growth of the stable phase becomes more collective and spatially diffuse, and the significance of a ‘critical nucleus’ is lost for deeper quenches